Graphite electrodes used in electric arc furnaces for the production of steel are normally prepared from needle like or premium grade cokes. The quality of coke, especially premium coke, is often measured by its coefficient of thermal expansion which preferably should not exceed 9.times.10.sup.-7 /.degree. C. and most preferably 2.times.10.sup.-7 /.degree. C. on a fine grained flour artifact. The electrodes usually are prepared from coke which contains a particle size distribution with a maximum size of about 1/2 inch down to a fine flour. In a one embodiment, coke particle sizes are from 10 to 50 percent by weight +20 mesh with at least 20 weight percent of the particles less than 40 mesh. The particle size distribution and structure of the petroleum coke raw material in the electrode are substantially retained through the graphitizing process. The resulting graphitized specimens can be examined by microscopic methods so that the final graphite product can in part be characterized by the particle size distribution and structure of the raw material.
In carrying out the electrode manufacturing process subdivided premium coke which has been calcined is mixed with a binder, usually a coal tar pitch, and a small percentage of iron oxide. The iron oxide is used to control the "puffing" of high sulfur petroleum coke during the subsequent electrode graphitization process. Small amounts of non-viscous petroleum oil may be added to the mixture as a lubricant. The plasticized mixture of sized coke, pitch and iron oxide is extruded at temperatures near the softening point of the pitch to form green electrodes of approximately the required finished dimensions. Usually these electrodes are from about 18 to about 24 inches in diameter and may be of varying lengths.
The green electrode next is baked at a temperature from about 1400.degree. to about 1800.degree. F. during which the binder is carbonized to form a rigid body. Subsequent to the baking process, the electrode may be impregnated (one or more times) with an impregnating pitch and rebaked to eventually provide a higher density and strength and lower electrical resistivity.
The final process step is that of graphitization. The baked carbon electrodes are packed in furnaces surrounded by insulating materials and heated to temperatures near 5000.degree. F. This temperature is necessary to convert the amorphous carbon in the electrode to the crystalline graphitic state.
The nature and quality of the binder used in the electrode manufacturing process is extremely important. Petroleum tars and pitches have not been heretofore proved useful for many reasons, such as the fact that electrodes so made are of uneven mechanical strength and are variable in electrical resistivity. Even highly aromatic tars resulting from cracking processes have failed to produce a satisfactory pitch by conventional means.
It is desirable to have sources of materials other than coal tar pitch for use as binders. It is particularly desirable to be able to use heavy oils, tars and other aromatic petroleum fractions for this purpose since these materials are readily available and often do not have other economically viable uses.
In accordance with this invention, binder pitch suitable for use in the preparation of graphite electrodes used in electric arc furnaces for the production of steel is obtained by hydrotreating an aromatic mineral oil, subjecting the hydrotreated material to thermal cracking, subjecting the thermal tar from the thermal cracking to vacuum distillation to recover a heavier thermal tar and combining the heavier thermal tar with super finely divided particles of calcined premium coke. In one aspect of the invention, the heavier thermal tar is subjected to a heat soak prior to combining it with the super fine particles of coke.